1. Field of the Invention
This invention relates to a novel method of electrodeposition coating, particularly a method of electrodeposition coating which permits low-temperature curing, and to a water-base paint resin composition which can be used advantageously in the above method.
2. Description of the Prior Art
Previously proposed methods of electrodeposition coating which form coated films capable of being cured at low temperatures of about 120.degree. to 140.degree. C. include, for example, those in which resins containing both a cationic group and a blocked isocyanate functional group capable of being dissociated at low temperatures are used (U.S. Pat. Nos. 3,984,299 and 4,435,559), and those in which resins containing both a cationic group and an N-methylol functional group are used (U.S. Pat. Nos. 4,341,676 and 4,501,833). Coated films formed by low-temperature curing in these methods are insufficient in adhesiveness and corrosion resistance.
It is an object of this invention to provide an electrodeposition coating method which gives excellent low-temperature curability, bath stability, surface smoothness and film properties (adhesion, corrosion resistance, etc.) by using an electrodeposition paint comprising a resin containing both an ionic group and a hydroxyl group, and a crosslinking agent containing alkoxysilane groups and optionally, hydroxysilane groups, which is of quite a different type from the vehicles previously used, as a film-forming vehicle.
Attempts have been made previously to improve the adhesion, weatherability and corrosion resistance of electrodeposited coated films by using a compound containing a hydroxysilane or an alkoxysilane group as a pre-treating agent (see, for example, U.S. Pat. Nos. 3,785,946 and 4,330,446). It is well known in the field of moisture-curable paints to utilize the condensation reaction of the hydroxysilane group and/or the alkoxysilane group for curing of coated films (for example, Japanese Laid-Open Patent Publications Nos. 73998/1977 and 40893/1979). As another use of an aqueous dispersion of the resin containing a hydroxysilane group and/or an alkoxysilane group, it was proposed to utilize a resin obtained by copolymerizing a vinyl or acrylic monomer containing the aforesaid functional group as a comonomer component for surface-treating inorganic building materials (Japanese Laid-Open Patent Publication No. 57860/1981).
An aqueous dispersion of such a resin containing a hydroxysilane group and/or an alkoxysilane group as a functional group was not previously thought to exist stably in a non-gelled state because of the high reactivity of the functional group. Hence, no attempt has been made to apply it to the field of electrodeposition coating. However, surprisingly in view of the prior thought, the present inventor has found that a crosslinking agent containing the above functional group and a resin containing both an ionic group and a hydroxyl group exists stably without gellation in their aqueous co-dispersion and this aqueous co-dispersion is useful for electrodeposition coating, and that when an electrodeposited film from an electrodeposition coating bath comprising this aqueous co-dispersion is heated after deposition, condensation reaction proceeds with the evaporation of water and the melting of the film whereby the film is crosslinked to form a uniform coated film excellent in film properties such as adhesion, weatherability and corrosion resistance. This discovery has led to the accomplishment of the present invention.